A conventional laser apparatus generates a laser beam mainly by converting an electric energy into a form of light (turns on a lamp) or an electric discharge to pump a laser medium. It is known that such method comprises a plurality of energy conversion processes and having a low-energy efficiency (not more than a few percent). The reason may include the fact that an electric energy, which has originally high quality, is converted into light through a low-efficient energy conversion in use.
In this regard, solar light can be used as a light source for laser oscillation without using the low-efficient electric-to-light conversion process. Only the principle of a solar light pumped laser has hitherto been studied mainly in research institutes and laboratories of universities. However, studies about a solar light pumped laser aiming at practical development or practical use level has been scarcely done.
Since the conventional solar light pumped laser apparatus pumps a laser medium by focusing solar light on the laser medium by using a reflection mirror or a lens, the apparatus cannot be made large and hence, there is a difficulty in the industrial application thereof. Furthermore, though various experimental studies have been reported, an imitation solar light sources are used in all the reported solar light pumped lasers with high efficiencies of more than 10 percent. Accordingly, it can be said that there has not been performed any studies about laser oscillation using actual solar light as a pumping light source. In order to utilize the solar light as a pumping light source for the practical use of the solar light pumped laser, the energy density of the solar light needs to be enhanced so as to generate a population inversion at a laser oscillation level. In this case, there is a problem in that the configuration of the reflection mirror and the lens for enhancing the energy density of the solar light must be examined.
On the other hand, literatures related to the solar light pumped laser, for example, Japanese Patent Laid-Open No. 2003-188441 (Patent Literature 1), Japanese Patent Laid-Open No. 2003-012569 (Patent Literature 2), Japanese Patent Laid-Open No. 2002-255501 (Patent Literature 3) and Japanese Patent Laid-Open No. H7-240553 (Patent Literature 4), have been known so far. Patent Literature 1 discloses a solar light pumped laser having a structure in which a coolant flows around a laser medium, yet does not disclose at any point of realizing an energy density necessary for highly efficient laser oscillation by using solar light of a low-energy efficiency.
FIG. 9 depicts the cross section of the conventional solar light pumped laser that has hitherto been proposed. The solar light pumped laser 100 shown in FIG. 9 comprises in general the Fresnel lens 102, the containing frame 104 provided with the reflection optical system for reflecting solar light focused by the Fresnel lens 102, and the laser medium 106. The total reflection mirror 108 is disposed on one end of the laser medium 106 and the half mirror 110 is disposed on the other end thereof to irradiate a laser beam in a direction indicated by the arrow X. The containing frame 104 retains air as a coolant internally. As shown in FIG. 9, the pumping solar light is entered into the laser medium 106 from one surface thereof. Since the total reflection mirror 108 is disposed on this surface of incidence, the laser medium 106 cannot be pumped efficiently in the traveling direction of the laser beam. Further, in this configuration, a large area ratio of cross sections between the opening of the containing frame 104 into which the solar light is entered and the laser medium 106 cannot be attained, there is an essential restriction in that an upper limit of a power amplification effect from solar light power to a laser beam is limited by this area ratio thereof. Further, solar light is hardly focused by the reflection elements disposed inside the containing frame 104, and irradiated onto the laser medium as scattered light. Accordingly, in this configuration, the laser medium 106 cannot be pumped efficiently in the laser oscillation direction. In addition, since air is used as a coolant, the laser medium is hardly expected to be cool, and this configuration cannot cope with an essential problem of efficiently cooling the laser medium and the Fresnel lens 102 which are served as a focusing optical system.
That is to say, the laser system disclosed by the above prior art uses or simply appropriates a commercially available laser cooling method, and lacks a viewpoint of the practical use of the method for focusing the solar light on the laser medium. Further, the lens and the reflection mirror included in this laser system are inappropriately arranged suitable for the industrial use of the solar light pumped laser. In addition, the laser system employs an asymmetrical optical arrangement in which pumping light propagates in an axial direction of a laser material, and this causes a problem in that homogeneous pumping of the material is difficult, resulting in a low efficiency.    [Patent Literature 1] Japanese Patent Laid-Open No. 2003-188441    [Patent Literature 2] Japanese Patent Laid-Open No. 2003-012569    [Patent Literature 3] Japanese Patent Laid-Open No. 2002-255501    [Patent Literature 4] Japanese Patent Laid-Open No. H7-240553